High intensity arc lamps are devices that emit a high intensity beam. The lamps generally include a gas containing chamber, for example, a glass bulb, with an anode and cathode that are used to excite the gas (ionizable medium) within the chamber. An electrical discharge is generated between the anode and cathode to provide power to the excited (e.g. ionized) gas to sustain the light emitted by the ionized gas during operation of the light source.
FIG. 1 shows a pictorial view and a cross section of a low-wattage parabolic prior art Xenon lamp 100. The lamp is generally constructed of metal and ceramic. The fill gas, Xenon, is inert and nontoxic. The lamp subassemblies may be constructed with high-temperature brazes in fixtures that constrain the assemblies to tight dimensional tolerances. FIG. 2 shows some of these lamp subassemblies and fixtures after brazing.
There are three main subassemblies in the prior art lamp 100: cathode; anode; and reflector. A cathode assembly 3a contains a lamp cathode 3b, a plurality of struts holding the cathode 3b to a window flange 3c, a window 3d, and getters 3e. The lamp cathode 3b is a small, pencil-shaped part made, for example, from thoriated tungsten. During operation, the cathode 3b emits electrons that migrate across a lamp arc gap and strike an anode 3g. The electrons are emitted thermionically from the cathode 3b, so the cathode tip must maintain a high temperature and low-electron-emission to function.
The cathode struts 3c hold the cathode 3b rigidly in place and conduct current to the cathode 3b. The lamp window 3d may be ground and polished single-crystal sapphire (AlO2). Sapphire allows thermal expansion of the window 3d to match the flange thermal expansion of the flange 3c so that a hermetic seal is maintained over a wide operating temperature range. The thermal conductivity of sapphire transports heat to the flange 3c of the lamp and distributes the heat evenly to avoid cracking the window 3d. The getters 3e are wrapped around the cathode 3b and placed on the struts. The getters 3e absorb contaminant gases that evolve in the lamp during operation and extend lamp life by preventing the contaminants from poisoning the cathode 3b and transporting unwanted materials onto a reflector 3k and window 3d. The anode assembly 3f is composed of the anode 3g, a base 3h, and tubulation 3i. The anode 3g is generally constructed from pure tungsten and is much blunter in shape than the cathode 3b. This shape is mostly the result of the discharge physics that causes the arc to spread at its positive electrical attachment point. The arc is typically somewhat conical in shape, with the point of the cone touching the cathode 3b and the base of the cone resting on the anode 3g. The anode 3g is larger than the cathode 3b, to conduct more heat. About 80% of the conducted waste heat in the lamp is conducted out through the anode 3g, and 20% is conducted through the cathode 3b. The anode is generally configured to have a lower thermal resistance path to the lamp heat sinks, so the lamp base 3h is relatively massive. The base 3h is constructed of iron or other thermally conductive material to conduct heat loads from the lamp anode 3g. The tubulation 3i is the port for evacuating the lamp 100 and filling it with Xenon gas. After filling, the tabulation 3i is sealed, for example, pinched or cold-welded with a hydraulic tool, so the lamp 100 is simultaneously sealed and cut off from a filling and processing station. The reflector assembly 3j consists of the reflector 3k and two sleeves 3l. The reflector 3k may be a nearly pure polycrystalline alumina body that is glazed with a high temperature material to give the reflector a specular surface. The reflector 3k is then sealed to its sleeves 3l and a reflective coating is applied to the glazed inner surface.
During operation, the anode and cathode become very hot due to electrical discharge delivered to the ionized gas located between the anode and cathode. For example, ignited Xenon plasma may burn at or above 15,000 C, and a tungsten anode/cathode may melt at or above 3600 C. degrees. The anode and/or cathode may wear and emit particles. Such particles can impair the operation of the lamp, and cause degradation of the anode and/or cathode.
One prior art sealed lamp is known as a bubble lamp, which is a glass lamp with two arms on it. The lamp has a glass bubble with a curved surface, which retains the ionizable medium. An external laser projects a beam into the lamp, focused between two electrodes. The ionizable medium is ignited, for example, using an ultraviolet ignition source, a capacitive ignition source, an inductive ignition source, a flash lamp, or a pulsed lamp. After ignition the laser generates plasma, and sustains the heat/energy level of the plasma. Unfortunately, the curved lamp surface distorts the beam of the laser. A distortion of the beam results in a focal area that is not crisply defined. While this distortion may be partially corrected by inserting optics between the laser and the curved surface of the lamp, such optics increase cost and complexity of the lamp, and still do not result in a precisely focused beam.
Compared with reflectorized small dimension concave and convex surfaces, lenses can be made more accurate and precise in their properties, as lenses can be ground and polished to much higher precision than a chamber interior reflective surface. On the other hand, mirrored surfaces may be more economical to produce in large assemblies where the surface defects are small in comparison to the aperture of the optics or the wavelengths under consideration.
When igniting laser plasma at low laser power, it is desirable that the focal spot of the laser is as small as possible and the numerical aperture (NA) is a large as possible. For example, such benefits are challenging when using the lamp 1600 of FIG. 16A. The parabolic mirror 1624 forming the interior surface of the lamp chamber 1620 is utilized to focus the laser light 1662 passing through the folding mirror 1680 and either high power or ignition aids are needed to ignite the laser plasma located at the focal region 1621. Practical implementations of parabolic mirrors suffer either from distortion from the optimal shape to surface roughness affecting the waist size of the laser beam 1662 size and shape. The folding mirror 1680 reflects the high intensity egress light 1629 toward a desired target, or toward intermediate optics (not shown).
Another implementation shown in FIG. 16B with a parabolic mirror adds a lens 1670 to focus the laser beam 1662 directly to the focal region 1621, allowing for a smaller plasma region. However, this lamp suffers from the low NA of the laser source, as the focal length of the lens needed in this implementation is fairly long. Therefore, there is a need to address one or more of the above mentioned shortcomings.